Abstract: Modular multilevel converter-railway static power conditioner (MMC-RPC) can effectively solve the power quality problems in the traction power supply system. However, the influence of low-frequency oscillation (LFO) in the vehicle-grid coupling system with the access of the MMC-RPC has not been clear. To solve this problem, this paper first considers the frequency coupling effect, and establishes the sequence admittance models of the CRH3, the CRH5 locomotive converters and the MMC-RPC based on the multi-harmonic linearization method, realizing the unification of different control strategies and converters' sequence admittance models in the single-phase stationary coordinate frame. Then the LFO of the vehicle coupling system with the access of the MMC-RPC is studied. According to the dominant phase margin and the sensitivity index, the influence of controller parameters on the system is evaluated quantitatively for the first time, and some parameter tuning criterions are proposed for the system stability improvement. Finally, based on the Matlab/Simulink simulation and the hardware-in-the-loop real-time simulation, the correctness of the stability analysis results is verified.